My 99 Altima's service engine soon light is ON after oil change and about 50 miles driving. This is the second time. Weird. Mechanic scanned error code, but it didn't return any specific error code. Is it possible? Mechanic searched internet and he said EGR valve need to be replaced. I don't know how he knew EGR valve problem without specific code.
What should I do? EGR valve must be changed?
Mechanic reset the code so light is OFF now but I think the light will be on next oil change. Please help me..
Thank you in advance.....

I would try checking for codes using the diagnostic mode screw on the ECM:

95–99

Turn ignition On.
Turn diagnostic mode selector on ECU fully clockwise and wait until inspection lamps flash. Number of flashes displayed indicates corresponding mode.
Note number of flashes, then immediately turn diagnostic mode selector fully counterclockwise.
If ignition is turned Off during diagnosis, in each mode, and then turned back on again after power to the ECU has dropped off completely, diagnosis will automatically return to Mode I.
For description of modes, proceed as follows:
(Mode I): with engine stopped, system in bulb check state. with engine running, system in malfunction warning state.
(Mode II): with engine stopped, system in self diagnostic results state. with engine running, system in front O2S monitor state.

Thank you for your reply.
The mechanic scanned my car and finally got "EGR control solenoid" code.
Is this different from EGR valve? And Do I have to replace (costs $200)EGR control solenoid ASAP?
If not, it will be get severe problem in the future?
Otherwise, I want to try clean it for myself.
How can I clean it?
Thank you again..

id check every rubber hose in the egr system first. every time ive seen a failure involving the egr system, theres been a collapsed and gummy hose somewhere in it. usually underneath the solenoid where the most heat is coming from. grab the hose and pinch it - if it feels squishy, replace it.

Thank you. My car broke down last saturday. Alternator burned...
Replaced alternator and battery. Is this caused by SEL or just coincidence?
Today, I checked the hose and one (short and goes to engine part) of them has 2 white lines and there is a thing(like a bean) between the white lines and it looks like blocking the flow. I took out that hose and ckecked the air flow by blowing breath. I felt air flow penerates the hose.
That little thing in the hose is normal (factory installed) or should I remove?
Thank you